Worldwide, 13 million infants are born preterm each year. In the United States, approximately 1 in 8 live births are preterm. The economic burden from prematurity is enormous costing over 28 billion dollars a year. These costs are not just for immediate neonatal care but also for long term care of these preterm children who are at increased risk for a spectrum of medical and neurobehavioral disorders. Currently, we have no effective strategies to predict or to prevent the majority of preterm births (PTBs). Based on the current model, key events in the pathogenesis of PTB are uterine infection and contractility. Assuming this model to be valid, the only strategies to decrease the risk for PTB are to monitor for increased uterine contractility and/or to treat the presumed underlying infectious process. Unfortunately, clinical trials attempting to treat or prevent uterine contractions and/or to treat infections have failed. Based in part on new findings linking, for example, dysbiosis of microbial communities (microbiota) inhabiting the human gastrointestinal tract to disease states through immune responses at the mucosal barriers, we propose a revised paradigm for the pathogenesis of PTB. Whereas PTB result from microbiota dysbiosis in the cervicovaginal (CV) compartment leading to a local immune response and altered tissue (cervix) homeostasis. The dysregulated immune response, mediated by the CV epithelial barrier, promotes changes in the cervix which leads to PTB. Furthermore, we propose that certain factors (e.g. behavior, social, environmental) known to mildly increase the risk for PTB, actually serve to alter the CV microbiota; thus, contributing to the pathogenesis of PTB. We hypothesize that microbial communities in the CV space will be stably composed of Lactobacillus spp. throughout pregnancy in women who are destined to have a term delivery. However, we propose that microbial communities will be less stable and enter states depleted of Lactobacillus spp. in women destined to have a PTB. Furthermore, specific microbial communities will induce an unfavorable immune response from the CV mucosal barrier; thus, microbial communities will be found to play a mechanistic role in PTB. This new paradigm will dramatically alter clinical care by allowing the identification, through the characterization of the CV microbiota, of the majority of women at risk-months prior to a clinical event- thus, providing opportunities for low-risk, feasible interventions. We have assembled an interdisciplinary team of experts in the field of reproductive medicine, microbiology, and perinatal nursing. We propose to assemble a prospective cohort, enriched with women with a prior PTB, that will allow for 3 longitudinal assessments during the 2nd and early 3rd trimester. At each study time point, we will determine the CV microbiota, the CV host immune response and determine if there are any molecular changes in the cervix consistent with premature cervical remodeling. In addition, we will comprehensively assess if behavioral, social and/or environmental factors modify the CV microbiota and/or its association with PTB.